(1) Field of the Invention
The present invention relates to a load detecting apparatus using a load cell type load detector (hereinafter simply referred to as "load cell"), and more particularly to a load detecting apparatus using a load cell in which the temperature compensation is effected for variations of a zero point of the load cell caused by temperature changes.
(2) Description of the Related Art
It has been known that, in a load detecting apparatus using a load cell, the zero point of the load cell varies depending on the temperature changes of the load cell itself caused by changes in an environment temperature. For the compensation of the variations of the zero point caused by the temperature changes, a prior art disclosure is found in each of Japanese Patent Application Kokai Publication No. Sho 54-55474 and Japanese Patent Application Kokai Publication No. Hei 1-152317.
The method with which the measured value is compensated by taking the temperature changes into consideration has a problem in that the method is very complicated. Also, the method in which the influence from temperature changes is attempted to be reduced by adding such an element as a temperature compensation element to a circuit such as a bridge circuit has a problem in that, when there is a large extent of temperature changes, the temperature compensation of the measured values to within a limited value not only involves technical difficulties but also complicates the circuit concerned, thus increasing the cost involved.
The Japanese Patent Application Kokai Publication No. Sho 54-55474 discloses a technique which, with a comparatively simple arrangement and with an object of ensuring to overcome the influence of temperature changes, attempts to maintain a constant temperature of a load cell by providing heat sources and heat sensitive elements in the neighborhood of the periphery of a case of the load cell and controlling temperatures of the heat sources by the heat sensitive elements.
In the arrangement disclosed in the Japanese Patent Application Kokai Publication No. Hei 1-152317 and shown in FIG. 1, the load cell 22 supported on the mounting base is covered by a cover 20 and there are provided a plurality of air conditioning means 21 for adjusting the temperature in the spaces formed by the cover 20 and a plurality of temperature measuring elements 23 for measuring the temperatures of the load cell 22, the mounting base, etc. This technique disclosed in the Japanese Patent Application Kokai Publication No. Hei 1-152317 attempts to realize a load detecting apparatus with which errors caused by a temperature drift is reduced by controlling the air conditioning means 21 such that the respective temperatures measured by the temperature measuring elements 23 become constant. This technique is to prevent the changes in the voltages outputted from the load cell which occur when the temperatures of the load cell are changed in the state wherein the constant load is being applied. The arrangement disclosed in the Japanese Patent Application Kokai Publication No. Hei 1-152317 attempts to prevent the temperature drift by maintaining a constant temperature higher than environment temperature changes in order to eliminate the influence of such environment temperature changes.
In both the above prior art techniques, it is attempted to compensate the changes in the measured values due to the temperature changes by controlling the temperatures of the load cell, and it is made possible to make the compensation only by the temperature control so that the compensation in this way is simpler than the compensation of the measured values themselves as practiced before.
However, in the arrangement disclosed in the Japanese Patent Application Kokai Publication No. Sho 54-55474, it is necessary that the load cell and the strain element which serves as a detecting element for the load cell be positioned in the space surrounded by an outer case, thermal insulating materials, etc. Also, in the arrangement disclosed in the Japanese Patent Application Kokai Publication No. Hei 1-152317, in addition to the necessity to position the load cell in the space surrounded by the cover, it was necessary to provide the air conditioning means. This means that, for purposes of the temperature compensation, a specific load cell must be manufactured so that the cost is high and its use becomes limited. Thus, there was a demand for a temperature compensation technique which provides an arrangement in which a general use load cell can be used as it is and which does not require any special or added installation.